As semiconductor devices are aggressively scaled down, the number of photoresist masking steps used in the photolithography process has significantly increased due to various etching and/or implanting requirements. Consequently, the number of post-masking cleaning steps has also increased. After a layer of photoresist is patterned on a semiconductor wafer and then subjected to a fabrication process, such as plasma etch or ion implantation, the patterned photoresist layer must be removed without leaving photoresist residue, which may detrimentally affect the resulting semiconductor device with respect to performance and reliability.
Traditionally, semiconductor wafers have been cleaned in batches by sequentially immersing the wafers into baths of different cleaning fluids, i.e., wet benches. However, with the advent of sub-0.18 micron geometries and 300 mm wafer processing, the use of batch cleaning has increased the potential for defective semiconductor devices due to cross-contamination and residual contamination. In order to mitigate the shortcomings of batch cleaning processes, single-wafer spin-type cleaning techniques have been developed. Conventional single-wafer spin-type cleaning apparatuses typically include a single fluid deliver line to dispense one or more cleaning fluids, such as de-ionized water, standard clean 1 (SC1) solution and standard clean 2 (SC2) solution, onto a surface of a semiconductor wafer in an enclosed environment.
With respect to single-wafer spin-type techniques, it has been found that the introduction of a reactive agent in the form of a gas, such as ozone, onto a surface of a spinning semiconductor wafer in addition to a cleaning fluid, e.g., de-ionized water, has been found to be highly effective in promoting oxidization, which assists in the removal of undesired material, such as photoresist, on the semiconductor wafer surface. A conventional method for introducing ozone involves mixing the ozone with the cleaning fluid and applying the mixture to the surface of the spinning semiconductor wafer. Another conventional method involves injecting the ozone into an enclosed cleaning chamber, where the spinning semiconductor wafer is being cleaned, to create an ozone environment. In this method, the ozone environment allows ozone to be diffused through a boundary layer of a cleaning fluid formed on the semiconductor wafer surface. The diffused ozone reacts with the undesired material on the wafer surface when the diffused ozone reaches the wafer surface. The boundary layer is maintained on the spinning semiconductor wafer surface by continuous application of the cleaning fluid.
A concern with the former conventional method for introducing ozone is that the concentration of ozone in an ozone-mixed cleaning fluid is typically very low, which results in a slow oxidation rate. As an example, the concentration of ozone in ozone-mixed de-ionized water is roughly 20 ppm at room temperature. Furthermore, the concentration of ozone is inversely proportional to temperature. Thus, if the ozone-mixed deionized water is heated, which may be preferred to increase the reaction rate on the semiconductor wafer surface, the ozone-mixed deionized water will have less concentration of ozone.
With respect to the latter conventional method, a concern is that ozone decays as the ozone diffuses through the boundary layer. The rate of ozone decay is dependent on the temperature of the boundary layer and the chemicals contained in the boundary layer. The ozone decay rate increases as the temperature of the boundary layer is increased. Thus, if the boundary layer is formed of heated cleaning fluid, such as heated deionized water, then the amount of ozone that can reach the semiconductor wafer surface for oxidation will be decreased due to the increased ozone decay rate caused by the higher temperature of the boundary layer. The ozone decay rate also increases significantly in certain chemical solutions, such as NH4OH, which is a highly desirable aqueous solution for cleaning semiconductor wafers. Thus, if the boundary layer is formed of NH4OH, then the amount of ozone that can reach the semiconductor wafer surface will be significantly decreased due to the increased ozone decay rate caused by the presence of NH4OH.
Another concern with the latter method is that a large amount of cleaning fluid and a high rotational speed of the semiconductor wafer are typically used to remove the by-products of oxidation during continuous reaction of ozone with the semiconductor wafer surface. The large amount of cleaning fluid results in a thick boundary layer, which reduces the amount of ozone that can reach the semiconductor wafer surface by diffusion. Furthermore, the high rotational speed tends to continuously push away the boundary layer containing the diffused ozone from the semiconductor wafer surface so that some of the diffused ozone does not have a chance to reach the semiconductor wafer surface for oxidation.
In view of the above-described concerns, there is a need for an apparatus and method for cleaning surfaces of semiconductor wafers using one or more cleaning fluids with reactive gaseous material, such as ozone, that can increase the amount of reactive gaseous agent that reaches the semiconductor wafer surface to promote a desired reaction, such as oxidation.